DISINFECTION by liaoqinmei

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									                                    CHAPTER 100
                                    DISINFECTION

101.   GENERAL

       Disinfection of the effluent shall be provided as necessary to meet applicable
       standards. The design shall consider meeting both the bacterial standards and the
       disinfectant residual limit in the effluent. The disinfection process should be
       selected after due consideration of waste characteristics, type of treatment process
       provided prior to disinfection, waste flow rates, pH of waste, disinfectant demand
       rates, current technology application, cost of equipment and chemicals, power
       cost, and maintenance requirements.

       Chlorine is the most commonly used chemical for wastewater disinfection. The
       forms most often used are liquid chlorine and calcium or sodium hypochlorite.
       Other disinfectants, including chlorine dioxide, ozone, bromine, or ultraviolet
       disinfection, may be accepted by the reviewing authority in individual cases. If
       halogens are utilized, it may be necessary to dehalogenate if the residual level in
       the effluent exceeds effluent limitations or would impair the natural aquatic
       habitat of the receiving stream.

       Where a disinfection process other than chlorine is proposed, supporting data
       from pilot plant installations or similar full scale installations may be required as a
       basis for the design of the system. Refer to Paragraph 53.2.

102.   CHLORINE DISINFECTION

       102.1 Type

              Chlorine is available for disinfection in gas, liquid (hypochlorite solution),
              and pellet (hypochlorite tablet) form. The type of chlorine should be
              carefully evaluated during the facility planning process. The use of
              chlorine gas or liquid will be most dependent on the size of the facility and
              the chlorine dose required. Large quantities of chlorine, such as are
              contained in ton cylinders and tank cars, can present a considerable hazard
              to plant personnel and to the surrounding area should such containers
              develop leaks. Both monetary cost and the potential public exposure to
              chlorine should be considered when making the final determination.
       102.2 Dosage

              For disinfection, the capacity shall be adequate to produce an effluent that
              will meet the applicable bacterial limits specified by the regulatory agency
              for that installation. Required disinfection capacity will vary, depending
              on the uses and points of application of the disinfection chemical. The
              chlorination system shall be designed on a rational basis and calculations
              justifying the equipment sizing and number of units shall be submitted for
              the whole operating range of flow rates for the type of control to be used.
              System design considerations shall include the controlling wastewater
              flow meter (sensitivity and location), telemetering equipment and
              chlorination controls. For normal domestic wastewater, the following may
              be used as a guide in sizing chlorination facilities.


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                Type of Treatment                                   Dosage
                Trickling filter plant effluent                    10 mg/L
                Activated sludge plant effluent                     8 mg/L
                Tertiary filtration effluent                        6 mg/L
                Nitrified effluent                                  6 mg/L

      102.3 Containers

               102.31   Cylinders

                        One hundred fifty pound (68 kg) cylinders are typically used
                        where chlorine gas consumption is less than 150 pounds per day
                        (68 kg/d). Cylinders should be stored in an upright position with
                        adequate support brackets and chains at 2/3 of cylinder height for
                        each cylinder.


               102.32   Ton Containers

                        The use of one-ton (907 kg) containers should be considered
                        where the average daily chlorine consumption is over 150
                        pounds (68 kg).

               102.33   Tank Cars

                        At large installations, the use of tank cars, generally
                        accompanied by evaporators, may be considered. Area wide
                        public safety shall be evaluated. No interruption of chlorination
                        shall be permitted during tank car switching.

                        The tank car being used for the chlorine supply shall be located
                        on a dead end, level track that is a private siding. The tank car
                        shall be protected from accidental bumping by other railway cars
                        by a locked derail device or a closed locked switch or both. The
                        area shall be clearly posted "DANGER-CHLORINE". The tank
                        car shall be secured by adequate fencing with gates provided
                        with locks for personnel and rail access.

                        The tank car site shall be provided with a suitable operating
                        platform at the unloading point for easy access to the protective
                        housing or the tank car for connection of flexible feedlines and
                        valve operation. Adequate area lighting shall be provided for
                        night time operation and maintenance.

               102.34   Liquid Hypochlorite Solutions

                        Storage containers for hypochlorite solutions shall be of sturdy,
                        non-metallic lined construction and shall be provided with secure
                        tank tops and pressure relief and overflow piping. Storage tanks


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                        should be either located or vented outside. Provision shall be
                        made for adequate protection from light and extreme
                        temperatures. Tanks shall be located where leakage will not
                        cause corrosion or damage to other equipment. A means of
                        secondary containment shall be provided to contain spills and
                        facilitate cleanup. Due to deterioration of hypochlorite solutions
                        over time, it is recommended that containers not be sized to hold
                        more than one month's needs. At larger facilities and locations
                        where delivery is not a problem, it may be desirable to limit on-
                        site storage to one week. Refer to Section 57.

               102.35   Dry Hypochlorite Compounds

                        Dry hypochlorite compounds should be kept in tightly closed
                        containers and stored in a cool, dry location. Some means of
                        dust control should be considered, depending on the size of the
                        facility and the quantity of compound used. Refer to Section 57.
      102.4 Equipment

               102.41   Scales

                        Scales for weighing cylinders and containers shall be provided at
                        all plants using chlorine gas. At large plants, scales of the
                        indicating and recording type are recommended. At least a
                        platform scale shall be provided. Scales shall be of corrosion-
                        resistant material.

               102.42   Evaporators

                        Where manifolding of several cylinders or ton containers will be
                        required to evaporate sufficient chlorine, consideration should be
                        given to the installation of evaporators to produce the quantity of
                        gas required.

               102.43   Mixing

                        The disinfectant shall be positively mixed as rapidly as possible,
                        with a complete mix being effected in 3 seconds. This may be
                        accomplished by either the use of turbulent flow regime or a
                        mechanical flash mixer.

               102.44   Contact Period and Tank

                        For a chlorination system, a minimum contact period of 15
                        minutes at design peak hourly flow or maximum rate of pumpage
                        shall be provided after thorough mixing. For evaluation of
                        existing chlorine contact tanks, field tracer studies should be
                        done to assure adequate contact time.

                        The chlorine contact tank shall be constructed so as to reduce
                        short-circuiting of flow to a practical minimum. Tanks not
                        provided with continuous mixing shall be provided with "over-


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                        and-under" or "end-around" baffling to minimize short-
                        circuiting.

                        The tank should be designed to facilitate maintenance and
                        cleaning without reducing effectiveness of disinfection.
                        Duplicate tanks, mechanical scrapers, or portable deck-level
                        vacuum cleaning equipment shall be provided. Consideration
                        should be given to providing skimming devices on all contact
                        tanks. Covered tanks are discouraged.

               102.45   Piping and Connections

                        Piping systems should be as simple as possible, specifically
                        selected and manufactured to be suitable for chlorine service,
                        with a minimum number of joints. Piping should be well
                        supported and protected against temperature extremes.
                        Due to the corrosiveness of wet chlorine, all lines designated to
                        handle dry chlorine shall be protected from the entrance of water
                        or air containing water. Even minute traces of water added to
                        chlorine results in a corrosive attack. Low pressure lines made
                        of hard rubber, saran-lined, rubber-lined, polyethylene,
                        polyvinylchloride (PVC), or other approved materials are
                        satisfactory for wet chlorine or aqueous solutions of chlorine.

                        The chlorine system piping shall be color coded and labeled to
                        distinguish it from other plant piping. Refer to Paragraph 54.5.
                        Where sulfur dioxide is used, the piping and fittings for chlorine
                        and sulfur dioxide systems shall be designed so that
                        interconnection between the two systems cannot occur.

               102.46   Standby Equipment and Spare Parts

                        Standby equipment of sufficient capacity should be available to
                        replace the largest unit during shutdowns. Spare parts shall be
                        available for all disinfection equipment to replace parts which are
                        subject to wear and breakage.
               102.47   Chlorinator Water Supply

                        An ample supply of water shall be available for operating the
                        chlorinator. Where a booster pump is required, duplicate
                        equipment should be provided, and, when necessary, standby
                        power as well. Protection of a potable water supply shall
                        conform to the requirements of Paragraph 56.2. Adequately
                        filtered plant effluent should be considered for use in the
                        chlorinator.

               102.48   Leak Detection and Controls

                        A bottle of 56 percent ammonium hydroxide solution shall be
                        available for detecting chlorine leaks. Where ton (907 kg)
                        containers or tank cars are used, a leak repair kit approved by the
                        Chlorine Institute shall be provided. Consideration should be

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                        given to the provision of caustic soda solution reaction tanks for
                        absorbing the contents of leaking one-ton (907 kg) containers
                        where such containers are in use. Consideration should be given
                        to the installation of automatic gas detection and related alarm
                        equipment.

      102.5 Housing

               102.51   Feed and Storage Rooms

                        If gas chlorination equipment or chlorine cylinders are to be in a
                        building used for other purposes, a gas-tight room shall separate
                        this equipment from any other portion of the building. Floor
                        drains from the chlorine room should not be connected to floor
                        drains from other rooms. Doors to this room shall open only to
                        the outside of the building, and shall be equipped with panic
                        hardware. Rooms shall be at ground level and should permit
                        easy access to all equipment.

                        Storage areas for one-ton (907 kg) cylinders should be separated
                        from the feed area. In addition, the storage area shall have
                        designated areas for "full" and "empty" cylinders. Chlorination
                        equipment should be situated as close to the application point as
                        reasonably possible. For additional safety considerations, refer
                        to Section 57.

               102.52   Inspection Window

                        A clear glass, gas-tight, window shall be installed in an exterior
                        door or interior wall of the chlorinator room to permit the units to
                        be viewed without entering the room.

               102.53   Heat

                        Rooms containing disinfection equipment shall be provided with
                        a means of heating so that a temperature of at least 60°F (16 °C)
                        can be maintained. The room should be protected from excess
                        heat. Cylinders shall be kept at essentially room temperature. If
                        liquid hypochlorite solution is used, the containers may be
                        located in an unheated area.

               102.54   Ventilation

                        With chlorination systems, forced, mechanical ventilation shall
                        be installed which will provide one complete fresh air change per
                        minute when the room is occupied. The entrance to the air
                        exhaust duct from the room shall be near the floor. The point of
                        discharge shall be so located as not to contaminate the air inlet to
                        any buildings or present a hazard at the access to the chlorinator
                        room or other inhabited areas. Air inlets shall be so located as to
                        provide cross ventilation with air and at such temperature that
                        will not adversely affect the chlorination equipment. The outside
                        air inlet shall be at least three feet above grade. The vent hose


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                        from the chlorinator shall discharge to the outside atmosphere
                        above grade. Where public exposure may be extensive,
                        scrubbers may be required on ventilation discharge.

               102.55   Electrical Controls

                        Switches for fans and lights shall be outside of the room at the
                        entrance. A labeled signal light indicating fan operation should
                        be provided at each entrance, if the fan can be controlled from
                        more than one point.

               102.56   Protective and Respiratory Gear

                        Respiratory air-pac protection equipment, meeting the
                        requirements of the National Institute for Occupational Safety
                        and Health (NIOSH), shall be available where chlorine gas is
                        handled, and shall be stored at a convenient location, but not
                        inside any room where chlorine is used or stored. Instructions
                        for using the equipment shall be posted. The units shall use
                        compressed air, have at least 30-minute capacity and be
                        compatible with the units used by the fire department responsible
                        for the plant.

       102.6 Sampling and Control

               102.61   Sampling

                        Facilities shall be included for sampling disinfected effluent after
                        the contact chamber as monitoring requirements warrant. In
                        large installations, or where stream conditions warrant,
                        provisions should be made for continuous monitoring of effluent
                        chlorine residual.

               102.62   Testing and Control

                        Equipment shall be provided for measuring chlorine residual
                        using accepted test procedures. The installation of demonstrated
                        effective facilities for automatic chlorine residual analysis,
                        recording, and proportioning systems should be considered at all
                        large installations.

                        Equipment shall also be provided for measuring fecal coliform
                        organisms using accepted test procedures as required by the
                        regulatory agency.

103.   DECHLORINATION

       103.1 Types

               Dechlorination of wastewater effluent may be necessary to reduce the
               toxicity due to chlorine residuals. The most common dechlorination
               chemicals are sulfur compounds, particularly sulfur dioxide gas or
               aqueous solutions of sulfite or bisulfite. Pellet dechlorination systems are
               also available for small facilities.

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               The type of dechlorination system should be carefully selected considering
               criteria including the following: type of chemical storage required, amount
               of chemical needed, ease of operation, compatibility with existing
               equipment, and safety.

      103.2 Dosage

               The dosage of dechlorination chemical should depend on the residual
               chlorine in the effluent, the final residual chlorine limit, and the particular
               form of the dechlorinating chemical used. The most common
               dechlorinating agent is sulfite. The following forms of the compound are
               commonly used and yield sulfite (SO2) when dissolved in water.

                                                               Theoretical mg/L Required
               Dechlorination Chemical                         to Neutralize 1 mg/L Cl2
               Sodium thiosulfate (solution)                                0.56
               Sodium sulfite (tablet)                                      1.78
               Sulfur dioxide (gas)                                          0.9
               Sodium meta bisulfite (solution)                             1.34
               Sodium bisulfite (solution)                                  1.46

               Theoretical values may be used for initial approximations, to size feed
               equipment with the consideration that under good mixing conditions 10%
               excess dechlorinating chemical is required above theoretical values.
               Excess sulfur dioxide may consume oxygen at a maximum of 1.0 mg
               dissolved oxygen for every 4 mg SO2.

               The liquid solutions come in various strengths. These solutions may need
               to be further diluted to provide the proper dose of sulfite.

      103.3 Containers
               Depending on the chemical selected for dechlorination, the storage
               containers will vary from gas cylinders, liquid in 50 gallon (190 L) drums
               or dry compounds. Dilution tanks and mixing tanks will be necessary
               when using dry compounds and may be necessary when using liquid
               compounds to deliver the proper dosage. Solution containers should be
               covered to prevent evaporation and spills.

      103.4 Feed Equipment, Mixing, and Contact Requirements

               103.41    Equipment

                         In general, the same type of feeding equipment used for chlorine
                         gas may be used with minor modifications for sulfur dioxide gas.
                         However, the manufacturer should be contacted for specific
                         equipment recommendations. No equipment should be
                         alternately used for the two gases. The common type of


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                        dechlorination feed equipment utilizing sulfur compounds
                        include vacuum solution feed of sulfur dioxide gas and a positive
                        displacement pump for aqueous solutions of sulfite or bisulfite.

                        The selection of the type of feed equipment utilizing sulfur
                        compounds shall include consideration of the operator safety and
                        overall public safety relative to the wastewater treatment plant's
                        proximity to populated areas and the security of gas cylinder
                        storage. The selection and design of sulfur dioxide feeding
                        equipment shall take into account that the gas reliquifies quite
                        easily. Special precautions must be taken when using ton (907
                        kg) containers to prevent reliquefaction.

                        Where necessary to meet the operating ranges, multiple units
                        shall be provided for adequate peak capacity and to provide a
                        sufficiently low feed rate on turn down to avoid depletion of the
                        dissolved oxygen concentrations in the receiving waters.
               103.42   Mixing Requirements

                        The dechlorination reaction with free or combined chlorine will
                        generally occur within 15 to 20 seconds. The dechlorination
                        chemical should be introduced at a point in the process where the
                        hydraulic turbulence is adequate to assure thorough and complete
                        mixing. If no such point exists, mechanical mixing shall be
                        provided. The high solubility of SO2 prevents it from escaping
                        during turbulence.

               103.43   Contact Time

                        A minimum of 30 seconds for mixing and contact time shall be
                        provided at the design peak hourly flow or maximum rate of
                        pumpage. A suitable sampling point shall be provided
                        downstream of the contact zone. Consideration shall be given to
                        a means of reaeration to assure maintenance of an acceptable
                        dissolved oxygen concentration in the stream following
                        sulfonation.
               103.44   Standby Equipment and Spare Parts

                        The same requirements apply as for chlorination systems. See
                        Paragraph 102.46.

               103.45   Sulfonator Water Supply

                        The same requirements apply as for chlorination systems. See
                        Paragraph 102.47.




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       103.5 Housing Requirements

               103.51   Feed and Storage Rooms

                        The requirements for housing SO2 gas equipment shall follow the
                        same guidelines as used for chlorine gas. Refer to Paragraph
                        102.5 for specific details.

                        When using solutions of the dechlorinating compounds, the
                        solutions may be stored in a room that meets the safety and
                        handling requirements set forth in Section 57. The mixing,
                        storage, and solution delivery areas must be designed to contain
                        or route solution spillage or leakage away from traffic areas to an
                        appropriate containment unit.

               103.52   Protective and Respiratory Gear
                        The respiratory air-pac protection equipment is the same as for
                        chlorine. See Paragraph 102.56. Leak repair kits of the type
                        used for chlorine gas that are equipped with gasket material
                        suitable for service with sulfur dioxide gas may be used. (Refer
                        to The Compressed Gas Association Publication CGA G-3-1995,
                        "Sulfur Dioxide") For additional safety considerations, see
                        Section 57.

       103.6 Sampling and Control

               103.61   Sampling

                        Facilities shall be included for sampling the dechlorinated
                        effluent for residual chlorine. Provisions shall be made to
                        monitor for dissolved oxygen concentration after sulfonation
                        when required by the regulatory agency.

               103.62   Testing and Control

                        Provision shall be made for manual or automatic control of
                        sulfonator feed rates based on chlorine residual measurement or
                        flow.

104.   ULTRAVIOLET RADIATION DISINFECTION

       Design standards, operating data, and experience for this process are not well
       established. Therefore, expected performance of the ultraviolet radiation
       disinfection (UVRD) units shall be based upon experience at similar full scale
       installations or thoroughly documented prototype testing with the particular
       wastewater. Critical parameters for UVRD units are dependent upon
       manufacturers’ design, lamp selection, tube materials, ballasts, configuration,
       control systems, and associated appurtenances. Proposals on this disinfection
       process will be reviewed on a case-by-case basis at the discretion of the reviewing
       authority under the provisions of Paragraph 53.2.
       Open channel designs with modular UVRD units that can be removed from the
       flow are required. At least two banks in series shall be provided in each channel

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       for disinfection reliability and to ensure uninterrupted service during tube
       cleaning or other required maintenance. Operator safety (electrical hazards and
       UV radiation exposure) and tube cleaning frequency shall be considered. The
       hydraulic properties of the system shall be designed to simulate plug flow
       conditions under the full operating flow range. In addition, a positive means of
       water level control must be provided to achieve the necessary exposure time.
       Also refer to Paragraphs 54.2 and 54.3. Closed chamber units will be reviewed
       on a case by case basis in accordance with Paragraph 53.2.

       This process should be limited to a high quality effluent having at least 65%
       ultraviolet radiation transmittance at 254 nanometers wave length, and BOD and
       suspended solids concentrations no greater than 30 mg/L at any time. The UV
       radiation dosage shall be based on the design peak hourly flow. As a general
       guide in system sizing for an activated sludge effluent with the preceding
       characteristics, a UV radiation dosage not less than 30,000 µW·s/cm2 may be
       used after adjustments for maximum tube fouling, lamp output reduction after
       8760 hours of operation, and other energy absorption losses.
       An alarm system shall be provided to separately indicate lamp failure and low UV
       intensity.

105.   OZONE

       Ozone systems for disinfection should be evaluated on a case-by-case basis.
       Design standards, operating data, and experience for this process are not well
       established. Therefore, design of these systems should be based upon experience
       at similar full scale installations or thoroughly documented prototype testing with
       the particular wastewater.




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